1. Field of the Invention
This invention relates to conductors and magnets for producing large magnetic fields, and more particularly to such magnets employing anisotropic superconductors where the field anisotropies in such superconductors are utilized to provide improved designs.
2. Description of Related Art
Superconductors of many types are known in the prior art, including both elemental metals and compounds of various types, such as oxides. The recent technical breakthrough reported by Bednorz and Mueller in Z. Phys. B, 64, 189 (1986) was the first major improvement in a superconducting material in the last decade. The materials of Bednorz and Mueller exhibited critical transition temperatures Tc that were substantially above the critical transition temperatures of materials previously known. In particular, Bednorz and Mueller described copper oxide materials including a rare earth element, or rare earth-like element, where the rare earth element could be substituted for by an alkaline earth element such as Ca, Ba, or Sr.
The work of Bednorz and Mueller has led to intensive investigation in many laboratories in order to develop materials having still higher Tc. For the most part, these high Tc oxide superconductors consist of compounds of La, Sr, Cu, and O, or compounds of Y, Ba, Cu, and O. A highlight of this activity was the attainment of superconductivity at temperatures of about 95xc2x0 K as reported ported by M. K. Wu et al and C. W. Chu et al, Phys. Rev. Lett. 58, 908 (1987). Later, Y1Ba2Cu3O7xe2x88x92x was isolated as the superconducting phase of these Yxe2x80x94Baxe2x80x94Cuxe2x80x94O mixed phase compositions, as reported by P. M. Grant et al, Phys. Rev. B, and R. J. Cava et al, Phys. Rev. Lett. 58, 1676 (1987). These materials have a layered perovskite structure comprising two dimensional CuO layers which are believed necessary for the attainment of high transition temperatures. Hidaka et al, Japanese J. Appl. Phys. 26, L377 (1987) reported upper critical field anisotropies of 5 in single crystals of La2xe2x88x92xBaxCuO4.
These superconducting materials are generally termed high Tc superconductors, and are materials having superconducting transition temperatures greater than 26xc2x0 K. This class of superconductors includes Cuxe2x80x94O planes separated by rare earth or rare earth-like elements and alkaline earth elements. The crystalline structure of these materials is now well characterized as reported in the above-cited technical papers.
High Tc superconductors of many forms have been prepared by a variety of techniques, including standard ceramic processing of oxide, carbonate, nitrate, powders, etc. to form of bulk materials, vapor transport for depositing thin films, and plasma spray coating. A copending application of P. Chaudhari et al, Ser. No. 027,584, filed Mar. 18, 1987 and assigned to the present assignee, describes a technique for producing thin films of these high Tc superconductors. Another copending application to J. Cuomo et al, Ser. No. 043,523, filed Apr. 28, 1987 and assigned to the present assignee now abandoned in favor of a continuing application Ser. No. 276,085 filed Nov. 23, 1988); describes a plasma spray coating technique for depositing these high Tc superconductors. More recently, epitaxial single crystal films have been reported by P. Chaudhari et al in a paper submitted to Phys. Rev. Lett.
Thus, significant technical achievements have been made in the science of superconducting materials in order to provide materials which exhibit critical transition temperatures above liquid nitrogen temperature (77xc2x0 K). However, applications of these materials, being obviously desireable, have not yet been possible. As will be seen, the invention herein is an application of these materials to the design of improved superconducting magnets, and is based on a discovery of the present applications that these high Tc superconductors can exhibit a significant critical magnetic field anisotropy and high critical currents.
Superconducting magnets are known in the art, and are conventionally used when large magnetic fields are to be produced. In fact, a great deal of speculation has occurred about the use of high Tc materials for high field magnets for such diverse applications as nuclear fusion, nuclear magnetic resonance (NMR) imaging, and vehicle propulsion systems. Generally, in order to manufacture a useful magnet, the superconductor must satisfy two criteria: (1) it must have a high upper critical field Hc2 so that the superconductor does not lose its zero resistance due to the field produced in the windings by the current through other windings, and (2) it must have a high critical current so that the magnetic field it creates is large. With traditional superconducting materials (i.e., non high Tc materials) the upper critical field is a composition-dependent property. However, high critical current in the presence of large magnetic fields is very dependent on the exact preparation techniques used to manufacture the material. Thus, high critical field and high critical current are not necessarily related to one another. Further, the initial studies on the new high Tc materials indicated that they exhibited a very high critical field but very low critical current. Thus, while the desireability of using these materials in magnets was apparent, it was not apparent that they could be successfully employed to make a good superconducting magnet. Still further, how one would implement them to make such a magnet was also not clear.
In their experimentation, applicants have discovered that these high Tc materials exhibit a very large critical field anisotropy and also exhibit a large critical current density along preferred directions. The nature of this anisotropy is that these materials can support large currents only in certain crystallographic planes. By proper design of the magnet windings, the current can be made to flow in the directions of large critical current, yet the field from the windings lies in directions of high critical field. This design will satisfy the two criteria previously described. Prior to the discovery of this large field anisotropy and the possibility of large critical currents, the design of an improved magnet was not possible. This was so even though small upper critical field anisotropies had been observed in some of these high Tc materials, as noted in the aforementioned Hidaka et al reference.
Accordingly, it is a primary object of the present invention to provide an improved design for a superconducting magnet.
It is an object of this invention to provide an improved design for a superconductor and for a superconducting magnet using high Tc superconductors for the magnet windings.
It is yet another object of this invention to provide an improved superconducting magnet, utilizing superconductors exhibiting significant critical field anisotropies, the design providing fields in the direction of high critical fields in the magnet windings.
It is yet another object of this invention to provide an improved superconducting magnet, utilizing superconductors exhibiting significant critical field anisotropies, and critical current anisotropies, the design providing fields in the direction of high critical fields with critical field anisotropy in the direction of high critical current.
It is another object of the present invention to provide improved superconducting toroids and solenoid-type magnets wherein the windings of these magnets are comprised of high Tc superconductors.
It is another object of this invention to provide an improved superconducting magnet comprised of high Tc superconductor materials as the windings of this magnet, where the windings are arranged with respect to the crystallographic current-carrying planes in these materials to provide high critical field and high critical current in the windings.
Superconducting magnets are described in which the windings are comprised of superconducting materials exhibiting critical field anisotropy, i.e., materials in which the critical field Hc2 is larger in one direction than in another direction. A large magnetic field anisotropy has been discovered in the high Tc superconductors, and it has also been found that these materials are capable of carrying high critical currents. In the practice of this invention, these factors are utilized to provide a design in which the current flows in the directions of high critical current and produces fields in the direction of high critical field. More specifically, the magnet windings are arranged so that the current direction through the windings is substantially parallel to the direction having the largest critical magnetic field. In particular, the current-carrying planes in these high Tc superconducting materials are arranged to be parallel to the direction in which the critical magnetic field Hc2 is largest so that the magnetic field H produced by supercurrents in the windings will be in a direction substantially parallel to the direction of maximum Hc2, if the windings are arranged as described in this invention.
The improved conductors and magnet windings can be comprised of a plurality of single crystals oriented in the same direction. Thin epitaxial films formed on flexible substrates are a particularly preferred embodiment to provide the magnet windings. Highly textured films, textured polycrystalline ceramics, etc. can also be utilized. A representative material for a superconductor winding in accordance with the present invention is a film or crystals of Y1Ba2Cu3O7xe2x88x92x, in which very large magnetic field anisotropies and high critical currents have recently been discovered.
These and other objects, features, and advantages will be apparent from the following more particular description of the preferred embodiments.